Well said and all true. But I think part of the problem certainly from William's point of view is that AE covered for C and C when they were meeting behind Diana's back for years, using the houses that AE was "doing up" to meet in secret. The appointment was widely seen as a reward, according to reports at the time, and was not open to others to apply for in an open way, put out to tender or whatever it was called.
There can be no dispute about her talent.
William's intense and has a long memory, and wouldn't want his stepmother's sister discussing doing up places with him.
Despite reports to the contrary I don't believe he likes her at all.
Of course it will seem hypocritical he's appointed his friend, but like AE he seems well-qualified to do the job. It's not the nepotism aspect for William, it's who she is and he would not want to work with her. Fresh start. Maybe she's happy to go as well?